Microwave filter

• US 4,074,214 A
• Filed: 09/20/1976
• Issued: 02/14/1978
• Est. Priority Date: 09/20/1976
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Aichholzer, Ronald A.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Nussbaum, Marvin
• Application Number:
  US05/724,501
• Time in Patent Office: 512 Days
• Field of Search: 333/73 S, 333/73 R, 333/84 R, 333/84 M, 333/73 C, 333/76, 333/73 W, 333/97 R, 333/82 R, 333/82 A, 333/82 B, 333/35, 333/96, 333/33, 331/96-102, 331/43, 325/445-447, 325/436-438, 325/473-474, 325/477, 325/460, 325/489, 325/388, 321/60, 321/69 R, 321/DIG. 1, 321/69 W, 321/69 NL, 329/160, 329/162
• US Class Current: 333/204
• CPC Class Codes: H01P 1/2039 Galvanic coupling between Input/Output

Abstract

A precisely controlled bandpass/bandstop filter is fabricated by coupling open circuit stubs to a transmission line. The first and second stubs are designed to resonate with the third stub at the center frequency of the passband. Both the first and third stubs are one quarter wavelength long at the center frequency of the stopband, thereby attenuating transmission line signals within this band. The second stub is of a predetermined dimension to attenuate signals at a flyback frequency which occurs within the stopband as a result of interreaction between the stubs.

In a particular application, the filter may be used to significantly enhance the signal to noise characteristic of the mixer portion of a radio receiver.

Claims

1. A transmission line filter for passing signals having frequencies within a passband centered about a first predetermined frequency and attenuating signals having frequencies within a stopband centered about a second predetermined frequency, the filter comprising:

a central transmission line having an input terminal and an output terminal for receiving signals at its input terminal and passing the same to said output terminal; and

first, second and third stubs, each stub of predetermined dimension and coupled to the transmission line at a predetermined location such that

a. the first and second stubs resonate with the third stub at the first predetermined frequency,

b. at least one of the first and third stubs attenuates transmission line signals having frequencies within the stopband, and

c. the third stub attenuates transmission line signals at a flyback frequency, which flyback frequency is created within the stopband by an interreaction between the stubs.

2. The filter of claim 1 wherein at least one of the first and third stubs has an electrical length of one-quarter wavelength at the second predetermined frequency.

3. The filter of claim 1 wherein the third stub has an electrical length of one-quarter wavelength at the second predetermined frequency, the filter further comprising a fourth stub having an electrical length equal to that of the third stub and coupled to the transmission line at approximately the same point as the third stub whereby the fourth stub provides additional attenuation to transmission line signals within the stopband.

4. The filter of claim 1 wherein the second predetermined frequency is the third harmonic of the first predetermined frequency.

5. The filter of claim 1 fabricated using microstrip techniques.

6. In a radio receiver wherein an RF signal is modulated by a local oscillator and a balanced diode mixer to an intermediate frequency for application to an IF stage, a pair of third harmonic filters, each filter comprising:

a transmission line adapted for coupling the local oscillator signal and the received RF signal to each diode; and

first, second and third stubs, each stub of predetermined dimension and coupled to the transmission line at a predetermined location such that

a. the first and second stubs resonate with the third stub to create a passband for passing both the local oscillator and received RF frequency signals,

b. at least one of the first and third stubs attenuates transmission line signals having frequencies within a stopband centered about the third harmonic of the local oscillator frequency, and

c. the third stub attenuates transmission line signals at a flyback frequency, which flyback frequency is created within the stopband by an interreaction between the stubs.

7. The harmonic filters of claim 6 wherein the electrical distance between the stubs and each diode is predeterminedly fixed such that a high impedance is reflected by the filter to the diode at the third harmonic local oscillator frequency.

8. The harmonic filters of claim 6 wherein at least one of the first and third stubs has an electrical length of one-quarter wavelength at the third harmonic of the local oscillator frequency.

9. The filters of claim 6 wherein each third stub has an electrical length of one-quarter wavelength at the third harmonic of the local oscillator frequency, each filter further comprising a fourth stub having an electrical length equal to that of the third stub and coupled to the transmission line at approximately the same point as the third stub whereby the fourth stub provides additional attenuation to transmission line signals within the stopband.

10. The filters of claim 6 fabricated using microstrip techniques.